Basic Subnetting Trick

This is good for Cisco exams, and work, if you ever need to figure out subnets on paper. 

To figure out the valid addresses in a subnet: 
Take the octet that is not 0 or 255, for example starting with 255.255.224.0, take 224, and subtract that value from 256 (256 – 224 = 32). Now make a chart starting at 0, and adding the value from that last step with each line, up to 256 (the note board I got for the CCNA had gridlines, which made it even easier) 


32 
64 
96 
128 
160 
192 
224 
256 


Now, leaving room in between, write on each line the value of the line below, minus 1 

 

 
  0       31 
 32       63 
 64       95 
 96       127 
128      159 
160      191 
192      223 
224      255 
256 


The values on the left are the valid subnet addresses, and the right is the broadcast address. 

If you want, now fill in the valid ranges in between 

 

Code:
  0         1-30          31 
  32      33-62          63 
  64      63-94          95 
  96      97-126        127 
128     127-158        159 
160     161-190        191 
192     192-222        223 
224     225-254        255 
256 


There you go, all your addresses, with subnet and broadcast addresses. 


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